Sound-absorbing panels



? June 24, 1958 M. c. JUNGER 2,840,179 SOUND-ABSORBING PANELS Filed June 17, 1954 INVENTOR. MIGUEL C. JUNGER ATTORNEYS United States Patent OfiFice SOUND-ABSORBIN G PANELS Miguel C. Junger, Cambridge, Mass. Application June 17, 1954, Serial No. 437,492 16 Claims. (Cl. 181-33) The present invention relates to building materials and more particularly to materials for the absorption and scattering of sound.

The most commonly used materials for this purpose are in the form of tiles to be attached to a wall or ceiling surface. They consist of rough-surfaced substances that absorb sound primarily by viscous action. Because they act by a viscous effect they are quite effective over a range of high frequencies, but relatively ineffective at frequencies of the order of two or three hundred cycles, such as the low-frequency components of speech or machine noise. Furthermore, these existing materials are often relatively heavy, which makes them expensive to attach and increases the risk of accidents from panels falling off. The tiles are often expensive to manufacture, requiring elaborate presses or dies.

It is therefore the primary object of the present invention to provide a cheap, light-weight, easily applied material to absorb sound within a room and to scatter and diffuse echoes.

It is another object of the present invention to provide a material which will absorb sound at both high and low frequencies. It is a further object of the invention to provide a material that can easily be attached to curved or irregular non-fiat ceilings.

In furtherance of these objects, it is a principal feature of the invention to make an acoustic material of a layer of irregularly-shaped cavities whereby the resonant properties of the cavities act to absorb sound at low frequencies and the irregular shape of the cavities acts to absorb sound at high frequencies. An additional feature is the fact that the irregular surface formed by the cavities prevents the setting up of flutter echoes at high frequencies.

A further feature of the invention is that the chief part of the absorption is done by the enclosed volume of air rather than by the substance of the material. For this reason, the layer of cavities can be made of extremely light material, and the possibility of accidents from falling panels is therefore almost eliminated.

Another feature of the invention is that because it is so light it can be easily attached. Merely gluing it to the wall or ceiling surface will hold it; no wood furring is necessary. Where the wall surface is irregular (as in the case of hangars or factories) and ordinary tiles could not be used, the material of the present invention is light enough to be supported by wire or cloth nettings below the ceiling.

Still another feature of the invention is that the materials that may be used in it are extremely inexpensive: papier-mache, pulp cardboard, pressed newspaper or the like. It is not necessary to mold the material into shape; screen processes can be used as in the manufacture of egg cartons.

These and other features of the invention will appear in the accompanying drawings in which Fig. l is an isometric view of one layer of the material;

2,840,179 Patented June 24, 1958 Fig. 2 is an isometric view of the second layer of the material;

Fig. 3 is a sectional view of the absorptive cavities attached to a wall surface; and

Fig. 4 is a sectional view of an alternative embodiment of the acoustic material attached to a wall surface.

The acoustic structure preferably consists of two sheets of material (as shown in Figs. 1 and 2) pressed or glued together (as shown in Figs. 3 and 4), although it could be made of a single sheet containing the resonant cavities, if such a molding operation were more convenient. Fig. 1 shows one of the two sheets that make up the material. It consists of a fiat piece 1 of papier-mache, fiberboard, or other soft and rough-surfaced material, into which a number of recesses or protrusions 3 have been formed. This can be done in the papier-mache manufacturing process simply by shaping the screen on which the pulp dries so that it leaves the desired shape. In a molding process, as for example, where the material was to be made from pressed scrap paper, the mold would be shaped to form the recesses.

The recesses 3 are shown in Fig. 1 in the shape of hollow truncated pyramids, but the shape is not particularly critical. The cavities should be shaped so that sound which enters them is not reflected out. The shape of the individual recesses should preferably be either conic or pyramidal so that a volume is enclosed within a number of non-parallel sides. Either a circle or any polygon may be used for the base shape. Another convenient shape in addition to the one shown in Figs. 1 and 2 is a recess with a hexagonal base that rises to a flat top, circular in cross-section. It is desirable that the recesses 3 have a flat top 4 for gluing them to the wall or ceiling surface. This flat area should be as small as possible, so that sound will not be reflected from it out of the cavity formed by the two recesses pressed together. For structural purposes it is preferable that the recesses 3 be interlaced on the sheet of material 1, rather than placed in rectilinear rows, so that the maximum resistance to bending will be obtained from the minimum of material.

Fig. 2 shows the other layer 2 for making the sound absorbing structure. It is shaped like the layer 1 in Fig. 1, with recesses 5 placed so that when the two sheets are pressed together with the protrusions outward, they will form resonant cavities out of the pairs of matched recesses. The layer shown in Fig. 2 differs from that in Fig. l in that the recesses 5 have holes 7 at their ends instead of the flat areas 4 of sheet 1.

The two layers are pressed together to form resonant cavities from the pairs of matched recesses as shown in Fig. 3. The fiat surfaces between the cavities are used for gluing the sheets together as shown at 6 and the flat surfaces 4 on sheet 1 are used to glue the material to a wall or ceiling surface 9 as shown at As suggested above, however, where the surface in question is irregular, the acoustic material could be supported from a netting. The holes 7 form entrances into the cavities which act as acoustic black bodies or as Helmholtz resonators, as explained below. Wads of some fibrous material such as rock wool or glass wool may be included in the cavities, as shown at 16, in order to increase the damping effect on air vibrations within the cavity.

Fig. 4 shows a modification by which the frequencies at which the cavities are effective may be lowered without increasing their volume. The material formed by pressing the sheets 1 and 2 together is glued at 8' to a wall surface 9', just as in Fig. 3. The holes 7 of the recess 5 are fitted with the tubes 11 extending part way into the resonant cavity. These tubes may be molded at the same time the recesses themselves are formed, or

they may be added in a second stage of manufacture. The effect of the tubes is to increase the mass of air vibrating at the mouth of the resonant cavity, when it acts as a Helmholtz resonator, and therefore to lower the resonant frequency. The acoustical characteristics can be further improved by placing wads of fibrous material 13 such as rock wool or glass fiber in thetubes it. This has the effect of broadening the resonant peak of the response curve by damping the vibration of the mass of air at its orifice.

The acoustic material shown in Figs. 3 and 4 acts in two major ways to absorb sound. First, the cavities contained in the material act as acoustical black bodies in that sound energy which enters the holes 7 or 7' is reflected back and forth within the cavity until it is spent, either by the absorptive action of the surfaceof the material itself or of the air Within the cavity or of the fibrous wad 10 or 13. This action is most effective at high frequencies; that is, frequencies greater than about 500 cycles per second. The absorptive action becomes more pronounced the higher the frequency, whereas ordinary viscous tiles become less or no more effective at very high frequencies (several thousand cycles per second). This black body action results entirely from the geometric shape of the cavities and is independent of their size or volume. It is necessary only that the cavity have as few parallel surfaces as possible so that the sound wave will be reflected indefinitely within the cavity.

A second effect takes place at low frequencies (under 500 cycles per second). The cavities act as Helmholtz resonators at a particular low frequency. That is, when sound of that frequency impinges on the orifice it causes a mass of air within the cavity and just behind the orifice to vibrate back and forth with a high amplitude, using the remaining air within the cavity as a kind of spring. Energy is absorbed by the vibrating mass as a result of its damping from contact with the sides of the cavity or with other air in the cavity and, in this Way, the impinging sound energy is absorbed.

The particular frequency f at which the resonator is eifectiveis given by the formula:

where the volume of the cavity is V, the cross-sectional area of the vibrating air mass is S, its volume v, and all dimensions are expressed in inches. It should be noted that the Helmholtz effect depends only on the volume of the cavity and is independent of its geometric shape; Thus, the low frequency Helmholtz action of the cavity is independent of the high frequency black body effect and vice versa.

vvhen the acoustic material is as shown in Fig. 3, the vibrating air mass is. a cylinder whose base is the hole '7 and whose height is .6 the radius of the hole. When the acoustic material is as shown in Fig. 4, the air mass is a cylinder whose base is the hole 7 and whose height is the length of the tube 11 plus .6 the radius of the base. It will be seen from the above formula that the presence of the tube lowers the frequency at which the cavity resonates without increasing the size of the cavities. As with other resonant phenomena, this Helmholtz effect operates over a band of frequencies near the resonant frequency. This band may be extended by increasing the damping by means of the fibrous material It) or 13. With the embodiment of Fig. 4, absorption of the order of 70% has been achieved at frequencies of two to four hundred cycles per second.

The acoustical material also acts in other ways to achieve sound absorption, insulation, or diffusion. First, the panels of material may vibrate at certain frequencies, thus absorbing the sound energy that causes the vicycles per second oration. Second, the layer of air enclosed between the top side of the sheet 1 or 1 and the wall surface 9 or 9 acts as an insulating air layer that tends to prevent the passage of sound through the wall into an adjoining room. This is the same result as that produced by expensive wood furring behind ordinary acoustic tiles. The same layer of dead air acts to improve the thermal insulation characteristics of the wall 9 or 9. Finally, the irregular surface presented by the material to the sound-emitting area produces scattering or diffusion of incident sound waves so that echoes are eliminated.

Having thus described my invention, I claim:

1. A sound absorbing and diffusing panel comprising a plurality volume-enclosing chambers, said panel material being shaped to form apertures in said chambers facing toward the sound-ernitting area, said chambers being so shaped that the cross-sectional areas of the enclosed volumes increase behind the apertures, tubes extending from the rims of the apertures part way into the chambers, and means for supporting the chambers in an i adjoining relation so as to form a continuous layer there- 2. A sound absorbing and diffusing panel comprising a plurality of volume-enclosing chambers, said panel material being shaped to form apertures in said chambers facing toward the sound-emitting area, said chambers being so shaped that the cross-sectional areas of the enclosed volumes increase behind the apertures, tubes extending from the rims of the apertures part way into the chambers, fibrous material within the tubes, and means for supporting the chambers in an adjoining relation so as to form a continuous layer thereof.

3. A sound absorbing and diffusing panel comprising a plurality of volume-enclosing chambers, said panel ma- 7 terial being. shaped to form apertures in said chambers facing toward the sound-emitting area, said chambers being so shaped that the cross-sectional areas of the enclosed volumes increase behind the apertures, tubes extending from the rims of the apertures part way into the chambers, fibrous material within the tubes, said chambers being formed of a number of non-parallel surfaces, and the chambers and tubes being dimensioned so as to produce a Helmholtz resonance at low frequencies, and means for supporting the chambers in an adjoining relation so as to form a continuous layer thereof.

4. A sound absorbing and diffusing panel comprising two sheets of material, each impressed with a series of protrusions positioned to form matching parts of volumeenclosing chambers and joining surfaces between the protrusions, said sheets being attached to each other along the joining surfaces to form said chambers, one of said sheets being shaped to form apertures at the ends of its protrusions, and the surfaces of the panel having the protrusions of the sheets.

5. A sound absorbing and diffusing panel as described in claim 4 in which the protrusions are tapered from the base to a smaller cross-sectional area at the top.

6. A sound absorbing and diffusing panel comprising two sheets of material, each impressed with a series of protrusions positioned to form matching parts of volumeenclosing chambers and joining surfaces between the protrusions, said sheets being attached to each other along the joining surfaces to form said chambers, the sheets being so shaped as to form apertures at the ends of the protrusions of the first sheet and flat surfaces at the ends of the protrusions of the second sheet for attaching the second sheet to a wall surface, the surfaces of the panel having the protrusions of the sheets.

7. A sound absorbing and diffusing panel as described in claim 6 in which the protrusions are tapered from the base to a smaller cross-sectional area at the top.

8. A sound absorbing and diffusing panel comprising two sheets of material, each impressed with a series of protrusions positioned to form matching parts of volumeenclosing chambers and joining surfaces between the protrusions, said sheets being attached to each other along the joining surfaces to form said chambers, one of said sheets being shaped to form apertures at the ends of its protrusions, and tubes extending from the rims of the apertures part way into the chambers.

9. A sound absorbing and diflusing panel comprising two sheets of material, each impressed with a series of protrusions positioned to form matching parts of volumeenclosing chambers and joining surfaces between the protrusions, said sheets being attached to each other along the joining surfaces to form said chambers, one of said sheets being shaped to form apertures at the ends of its protrusions, tubes extending from the rims of the apertures part way into the chambers, and fibrous material within the tubes.

10. A sound absorbing and diffusing panel as described in claim 9 in which the protrusions are tapered from the base to a smaller cross-sectional area at the top.

11. A sound absorbing and diffusing panel comprising two sheets of material, each impressed with a series of protrusions positioned to form matching parts of volumeenclosing chambers and joining surfaces between the protrusions, said sheets being attached to each other along the joining surfaces to form said chambers, one of said sheets being shaped to form apertures at the ends of its protrusions, and fibrous material within the chambers, the surfaces of the panel having the protrusions of the sheets.

12. A sound absorbing and diffusing panel as described in claim 11 in which the protrusions are tapered from the base to a smaller cross-sectional area at the top.

13. A sound absorbing and diffusing panel comprising two sheets of material, each impressed with a series of protrusions positioned to form matching parts of volumeenclosing chambers and joining surfaces between the protrusions, said sheets being attached to each other along the joining surfaces to form said chambers, one of said sheets being shaped to form apertures at the ends of its protrusions, tubes extending from the rims of the ape tures part way into the chambers, said chambers being formed of a number of non-parallel surfaces and the chambers and tubes being dimensioned so as to produce a Helmholtz resonance at low frequencies.

14. A sound absorbing and difiusing panel as described in claim 13 in which the protrusions are tapered from the base to a smaller cross-sectional area at the top.

15. A sound absorbing and diffusing panel comprising two sheets of sound-absorptive material, each impressed with a series of protrusions in the shape of truncated hexagonal pyramids and positioned to form matching parts of volume-enclosing chambers and joining surfaces between the protrusions, said sheets being attached to each other along the joining surfaces to form said chambers, said sheets being shaped to form apertures at the ends of the protrusions of one of the sheets and flat surfaces at the ends of the protrusions of the other, tubes extending from the rims of the apertures part way into the chambers, and fibrous material within the tubes, said chambers and tubes being dimensioned so as to produce a Helmholtz resonance at low frequencies.

16. A sound absorbing and difiusing panel comprising two sheets of sound-absorptive material, each impressed with a series of pyramidal protrusions tapering from a hexagonal cross-section at the base to a circular crosssection at the top and positioned to form matching parts of volume-enclosing chambers and joining surfaces between the protrusions, said sheets being attached to each other along the joining surfaces to form said chambers, said sheets being shaped to form apertures at the ends of the protrusions of one of the sheets and flat surfaces at the ends of the protrusions of the other, tubes extending from the rims of the apertures part way into the chambers, and fibrous material within the tubes, said chambers and tubes being dimensioned so as to produce a Helmholtz resonance at low frequencies.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Publication, The Journal of the Acoustical Society of America, January, 1952. 

